Ocean Springs Offer Hint Of How Corals Will Respond To Ocean Acidification

The density of coral skeletons will be reduced by ocean acidification due to rising carbon dioxide levels, according to an international group of scientists led by the University of California, Santa Cruz. The study, published in Proceedings of the National Academy of Sciences, focused on corals growing where submarine springs naturally lower the pH of seawater. The findings of this study are the first to show corals are not able to fully acclimate to low pH conditions in nature.

“People have seen similar effects in laboratory experiments,” said Adina Paytan, a research scientist at the Institute of Marine Sciences at UC Santa Cruz. “We looked in places where the corals are exposed to low pH for their entire life span. The good news is that they don’t just die. They are able to grow and calcify, but they are not producing robust structures.”

The pH level of the surface waters is being lowered as the world´s oceans continue to absorb more carbon dioxide from the steadily rising levels in the atmosphere. Although scientists do not expect the oceans to become literally acidic, the changes in seawater chemistry that move it closer to the acidic range of the pH scale are called ocean acidification. Payton and her colleagues studied coral reefs along the Caribbean coastline of Mexico´s Yucatan Peninsula where submarine springs decrease the pH level of the surrounding water in a localized, natural setting, similar to the widespread ocean acidification that is occurring as the oceans absorb increasing amounts of carbon dioxide from the atmosphere.

The research team, led by graduate student Elizabeth Crook, deployed instruments to monitor seawater chemistry around the springs and removed skeletal cores from colonies of Porites astreoides, an important Caribbean reef-building coral. Then, in the Woods Hole Oceanographic Institution (WHOI) lab of Anne Cohen, the team performed CT scans of the core samples to measure densities and determine annual calcification rates.

The team found coral calcification rates decrease significantly along a natural gradient in seawater pH. In addition, ocean acidification lowers the carbonate ion concentration in seawater, making it more difficult for corals to build their calcium carbonate skeletons.

“Carbonate ions are the building blocks they need to grow their skeletons. When the pH is lower the corals have to use more energy to accumulate these carbonate building blocks internally. As a result, the calcification rate is lower and they lay down less dense skeletons,” Paytan said.

Because of the reduced density, coral skeletons are more vulnerable to mechanical erosion during storms, organisms that bore into corals, and parrotfish, which sometimes feed on corals. This vulnerability could lead to a weakening of the reef framework, which would result in degradation of the complex coral reef ecosystem.

“There are likely to be major shifts in reef species and some loss of coral cover, but if ocean acidification is the only impact there won’t be total destruction,” Paytan said. “We need to protect corals from other stressors, such as pollution and overfishing. If we can control those, the impact of ocean acidification might not be as bad.”